In the following passage from The Perks of Being a Wallflower by Stephen Cbhosky, "english" isn't capitalized:
There is this one girl in my advanced english class named Susan. In middle school, Susan was very funny to be around. She liked movies, and her brother Frank made her tapes of this great music that she shared with us.
As far as I know, English is written with a capital E except for the billiards move. In this book, however, it is consistently written in lower case. That doesn't quite fit the main character's personality, but it still could be intentional (the author is trying to mimic teen speech).
Is this a mistake, or is there any reason why advanced english class shouldn't be written with a capital E?